Distinct characteristics of neonatal platelet reactivity

Comparison of platelet proteomic profiles between children and adults reveals origins of functional differences

BACKGROUND

Platelets are blood cells responsible for the prevention of blood loss upon vessel wall disruption. It has been demonstrated that platelet functioning differs significantly between adult and pediatric donors. This study aimed to identify potential differences between the protein composition of platelets of pediatric, adolescent, and adult donors.

METHODS

Platelet functional testing was conducted with live cell flow cytometry. Using a straightforward approach to platelet washing based on the sequential platelets centrifugation-resuspension, we were able to obtain stable and robust proteomics results, which corresponded to previously published data.

RESULTS

We have identified that pediatric donors' platelets have increased amounts of proteins, responsible for mitochondrial activity, proteasome activity, and vesicle transport. Flow cytometry analysis of platelet intracellular signaling and functional responses revealed that platelets of the pediatric donors have diminished granule secretion and increased quiescent platelet calcium concentration and decreased calcium mobilization in response to ADP. We could explain the observed changes in calcium responses by the increased mitochondria protein content, and the changes in granule secretion could be explained by the differences in vesicle transport protein content.

CONCLUSIONS

Therefore, we can conclude that the age-dependence of platelet functional responses originates from the difference in platelet protein content.

IMPACT

Platelets of infants are known to functionally differ from the platelet of adult donors, although the longevity and persistivity of these differences are debatable. Pediatric donor platelets have enhanced amounts of mitochondrial, proteasomal, and vesicle transport proteins. Platelets of the pediatric donors had increased cytosolic calcium in the resting state, what is explained by the increased numbers of mitochondrial proteins. Infants had decreased platelet granule release, which resolved upon adolescence. Thus, platelets of the infants should be assessed differently from adult platelets. Differences in platelet proteomic contents persisted in adolescent groups, yet, no significant differences in platelet function were observed.

Immature Platelet Fraction and Its Kinetics in Neonates

Thrombocytopenia is a common abnormality encountered in the neonatal period, and immature platelet fraction (IPF) may be an informative indicator of thrombopoiesis; however, data on IPF in neonates are scarce. To define reference intervals (RIs) and factors affecting IPF in neonates, we measured the IPF of 533 consecutive neonates. With a multiple regression analysis of 330 newborns with normal platelet counts at birth, premature delivery, neonatal asphyxia, intrauterine infection, chromosomal abnormalities, and respiratory disorders were identified as independent factors for IPF%. The RIs of IPF% and absolute IPF value in neonates were determined to be 1.3% to 5.7% and 3.2 to 14.5×10 9 /L, respectively. On day 14 after birth, IPF% increased to twice the value at birth and thereafter returned to the previous value on day 28. Reticulocyte counts, in contrast, were the lowest at day 14. IPF% was increased in 16 thrombocytopenic patients with various clinical conditions, especially those with immune-mediated thrombocytopenia. IPF in neonates may be evaluated essentially based on the same RIs as in adults, although some precautions must be taken when evaluating IPF in neonates in the first 2 weeks of life. IPF may be useful for evaluating thrombopoiesis and thrombocytopenia in neonates.

Direct chemical reprogramming of human cord blood erythroblasts to induced megakaryocytes that produce platelets

Reprogramming somatic cells into megakaryocytes (MKs) would provide a promising source of platelets. However, using a pharmacological approach to generate human MKs from somatic cells remains an unmet challenge. Here, we report that a combination of four small molecules (4M) successfully converted human cord blood erythroblasts (EBs) into induced MKs (iMKs). The iMKs could produce proplatelets and release functional platelets, functionally resembling natural MKs. Reprogramming trajectory analysis revealed an efficient cell fate conversion of EBs into iMKs by 4M via the intermediate state of bipotent precursors. 4M induced chromatin remodeling and drove the transition of transcription factor (TF) regulatory network from key erythroid TFs to essential TFs for megakaryopoiesis, including FLI1 and MEIS1. These results demonstrate that the chemical reprogramming of cord blood EBs into iMKs provides a simple and efficient approach to generate MKs and platelets for clinical applications.

The Vascular Endothelium and Coagulation: Homeostasis, Disease, and Treatment, with a Focus on the Von Willebrand Factor and Factors VIII and V

The vascular endothelium has several important functions, including hemostasis. The homeostasis of hemostasis is based on a fine balance between procoagulant and anticoagulant proteins and between fibrinolytic and antifibrinolytic ones. Coagulopathies are characterized by a mutation-induced alteration of the function of certain coagulation factors or by a disturbed balance between the mechanisms responsible for regulating coagulation. Homeostatic therapies consist in replacement and nonreplacement treatments or in the administration of antifibrinolytic agents. Rebalancing products reestablish hemostasis by inhibiting natural anticoagulant pathways. These agents include monoclonal antibodies, such as concizumab and marstacimab, which target the tissue factor pathway inhibitor; interfering RNA therapies, such as fitusiran, which targets antithrombin III; and protease inhibitors, such as serpinPC, which targets active protein C. In cases of thrombophilia (deficiency of protein C, protein S, or factor V Leiden), treatment may consist in direct oral anticoagulants, replacement therapy (plasma or recombinant ADAMTS13) in cases of a congenital deficiency of ADAMTS13, or immunomodulators (prednisone) if the thrombophilia is autoimmune. Monoclonal-antibody-based anti-vWF immunotherapy (caplacizumab) is used in the context of severe thrombophilia, regardless of the cause of the disorder. In cases of disseminated intravascular coagulation, the treatment of choice consists in administration of antifibrinolytics, all-trans-retinoic acid, and recombinant soluble human thrombomodulin.

Developmental hemostasis in the neonatal period

BACKGROUND

The hemostatic system is complex and evolves continuously since gestation and well into the adult years, in a process known as "developmental hemostasis."

DATA SOURCES

A comprehensive review was performed after an extensive literature search on PubMed/MEDLINE concerning developmental hemostasis during the neonatal period. Relevant cross references were also included.

RESULTS

Although part of a system, each component of the hemostatic system evolves differently, with many displaying both quantitative and qualitative age-related differences. This leads to drastic disparities between the coagulation system of neonates and both other children's and adults', while still maintaining a generally balanced and physiological hemostasis. The motives behind this process remain to be fully elucidated but may be, at least in part, related to non-hemostatic factors.

CONCLUSIONS

Knowledge regarding "developmental hemostasis" is essential for everyone caring for newborns or even children in general and in this review, we describe each hemostatic system component's neonatal characteristics and age-related progression as well as explore some of the possible physiological motives behind the process.

Age-Dependent Control of Collagen-Dependent Platelet Responses by Thrombospondin-1-Comparative Analysis of Platelets from Neonates, Children, Adolescents, and Adults

Platelet function is developmentally regulated. Healthy neonates do not spontaneously bleed, but their platelets are hypo-reactive to several agonists. The mechanisms underlying immature platelet function in neonates are incompletely understood. This critical issue remains challenging for the establishment of age-specific reference ranges. In this study, we evaluated platelet reactivity of five pediatric age categories, ranging from healthy full-term neonates up to adolescents (11–18 years) in comparison to healthy adults (>18 years) by flow cytometry. We confirmed that platelet hypo-reactivity detected by fibrinogen binding, P-selectin, and CD63 surface expression was most pronounced in neonates compared to other pediatric age groups. However, maturation of platelet responsiveness varied with age, agonist, and activation marker. In contrast to TRAP and ADP, collagen-induced platelet activation was nearly absent in neonates. Granule secretion markedly remained impaired at least up to 10 years of age compared to adults. We show for the first time that neonatal platelets are deficient in thrombospondin-1, and exogenous platelet-derived thrombospondin-1 allows platelet responsiveness to collagen. Platelets from all pediatric age groups normally responded to the C-terminal thrombospondin-1 peptide RFYVVMWK. Thus, thrombospondin-1 deficiency of neonatal platelets might contribute to the relatively impaired response to collagen, and platelet-derived thrombospondin-1 may control distinct collagen-induced platelet responses.

Platelet function and microvesicle generation in patients with hemophilia A

Our results do not support any effect of FVIII on platelet function in patients with severe HA treated under the regime of prophylaxis.

Shear-Induced Platelet Activation is Sensitive to Age and Calcium Availability: A Comparison of Adult and Cord Blood

INTRODUCTION

Antiplatelet therapy for neonates and infants is often extrapolated from the adult experience, based on limited observation of agonist-induced neonatal platelet hypoactivity and poor understanding of flow shear-mediated platelet activation. Therefore, thrombotic events due to device-associated disturbed flow are inadequately mitigated in critically ill neonates with indwelling umbilical catheters and infants receiving cardiovascular implants.

METHODS

Whole blood (WB), platelet-rich plasma (PRP), and gel-filtered platelets (GFP) were prepared from umbilical cord and adult blood, and exposed to biochemical agonists or pathological shear stress of 70 dyne/cm2. We evaluated α-granule release, phosphatidylserine (PS) scrambling, and procoagulant response using P-selectin expression, Annexin V binding, and thrombin generation (PAS), respectively. Activation modulation due to depletion of intracellular and extracellular calcium, requisite second messengers, was also examined.

RESULTS

Similar P-selectin expression was observed for sheared adult and cord platelets, with concordant inhibition due to intracellular and extracellular calcium depletion. Sheared cord platelet Annexin V binding and PAS activity was similar to adult values in GFP, but lower in PRP and WB. Annexin V on sheared cord platelets was calcium-independent, with PAS slightly reduced by intracellular calcium depletion.

CONCLUSIONS

Increased PS activity on purified sheared cord platelets suggest that their intrinsic function under pathological flow conditions is suppressed by cell-cell or plasmatic components. Although secretory functions of adult and cord platelets retain comparable calcium-dependence, PS exposure in sheared cord platelets is uniquely calcium-independent and distinct from adults. Identification of calcium-regulated developmental disparities in shear-mediated platelet function may provide novel targets for age-specific antiplatelet therapy.

Nano-scaled MTCA-KKV: for targeting thrombus, releasing pharmacophores, inhibiting thrombosis and dissolving blood clots in vivo

BACKGROUND

In vitro (1R,3S)-1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxyl-Lys(Pro-Ala-Lys)-Arg-Gly-Asp-Val (MTCA-KKV) adheres activated platelets, targets P-selectin and GPIIb/IIIa. This led to the development of MTCA-KKV as thrombus targeting nano-medicine.

METHODS

MTCA-KKV was characterized by nano-feature, anti-thrombotic activity, thrombolytic activity, thrombus target and targeting release.

RESULTS

In vivo 0.01 μmol/kg of MTCA-KKV formed nano-particles less than 100 nm in diameter, targeted thrombus, released anti-thrombotic and thrombolytic pharmacophores, prevented thrombosis and dissolved blood clots.

CONCLUSION

Based on the profiles of targeting thrombus, targeting release, inhibiting thrombosis and dissolving blood clots MTCA-KKV is a promising nano-medicine.

Heptapeptide-based modification leading to enhancing the action of MTCA on activated platelets, P-selectin, GPIIb/IIIa

AIM

The modification of platelet inhibitor to enhance its targeting capacity toward platelets is of clinical importance. Thus, (1R, 3S)-1-methyl-1, 2, 3, 4-tetrahydro-β-carboline-3-carboxylic acid (MTCA), a platelet inhibitor, was modified with Lys(Pro-Ala-Lys)-Arg-Gly-Asp-Val (KKV), platelet targeting peptide, to form MTCA-KKV.

MATERIALS & METHODS

MTCA and MTCA-KKV were synthesized to identify the effect of KKV modification on MTCA and platelets.

RESULTS

Atomic force microscopy imaged MTCA-KKV effectively accumulated on activated platelets. UV spectra showed that MTCA-KKV concentration dependently changed P-selectin and GPIIb/IIIa conformations. For platelet aggregation, the IC₅₀ of MTCA-KKV was approximately 1/10 folds of MTCA.

CONCLUSION

KKV modification led to forming MTCA-KKV that is superior to MTCA in terms of accumulating on activated platelets, targeting P-selectin and GPIIb/IIIa and inhibiting platelet aggregation. MTCA-KKV could be a promising lead for further investigation.

Significant Hypo-Responsiveness to GPVI and CLEC-2 Agonists in Pre-Term and Full-Term Neonatal Platelets and following Immune Thrombocytopenia

Neonatal platelets are hypo-reactive to the tyrosine kinase-linked receptor agonist collagen. Here, we have investigated whether the hypo-responsiveness is related to altered levels of glycoprotein VI (GPVI) and integrin α2β1, or to defects in downstream signalling events by comparison to platelet activation by C-type lectin-like receptor 2 (CLEC-2). GPVI and CLEC-2 activate a Src- and Syk-dependent signalling pathway upstream of phospholipase C (PLC) γ2. Phosphorylation of a conserved YxxL sequence known as a (hemi) immunotyrosine-based-activation-motif (ITAM) in both receptors is critical for Syk activation. Platelets from human pre-term and full-term neonates display mildly reduced expression of GPVI and CLEC-2, as well as integrin αIIbβ3, accounted for at the transcriptional level. They are also hypo-responsive to the two ITAM receptors, as shown by measurement of integrin αIIbβ3 activation, P-selectin expression and Syk and PLCγ2 phosphorylation. Mouse platelets are also hypo-responsive to GPVI and CLEC-2 from late gestation to 2 weeks of age, as determined by measurement of integrin αIIbβ3 activation. In contrast, the response to G protein-coupled receptor agonists was only mildly reduced and in some cases not altered in neonatal platelets of both species. A reduction in response to GPVI and CLEC-2, but not protease-activated receptor 4 (PAR-4) peptide, was also observed in adult mouse platelets following immune thrombocytopenia, whereas receptor expression was not impaired. Our results demonstrate developmental differences in platelet responsiveness to GPVI and CLEC-2, and also following immune platelet depletion leading to reduced Syk activation. The rapid generation of platelets during development or following platelet depletion is achieved at the expense of signalling by ITAM-coupled receptors.

Current status of blood 'pharming': megakaryoctye transfusions as a source of platelets

PURPOSE OF REVIEW

Donor-derived platelets have proven to be of hemostatic value in many clinical settings. There is a fear that the need for platelets may outgrow the donor pool in first-world countries. Moreover, there are other challenges with donor platelets that add to the impetus to find an alternative platelet source, especially after the megakaryocyte cytokine thrombopoietin was identified. Megakaryocytes have since been differentiated from numerous cell sources and the observed released platelet-like particles (PLPs) have led to calls to develop such products for clinical use. The development of megakaryocytes from embryonic stem cell also supported the concept of developing nondonor-based platelets.

RECENT FINDINGS

Several groups have claimed that nondonor-based platelets derived from in-vitro grown megakaryocytes may soon become available to supplement or replace donor-derived products, but their number and quality has been wanting. A possible alternative of directly infusing megakaryocytes that release platelets in the lungs - similar to that recently shown for endogenous megakaryocytes - has been proposed.

SUMMARY

This present review will describe the present state-of-the-art in generating and delivering nondonor-derived platelets. Progress has been slow, but advances in our ability to generate human megakaryocytes in culture, generate PLPs from these cells, and test the functionality of the resultant platelets in vitro and in vivo have identified important remaining challenges and raised alternative potential solutions.

Placenta and Placental Derivatives in Regenerative Therapies: Experimental Studies, History, and Prospects

Placental structures, capable to persist in a genetically foreign organism, are a natural model of allogeneic engraftment carrying a number of distinctive properties. In this review, the main features of the placenta and its derivatives such as structure, cellular composition, immunological and endocrine aspects, and the ability to invasion and deportation are discussed. These features are considered from a perspective that determines the placental material as a unique source for regenerative cell therapies and a lesson for immunological tolerance. A historical overview of clinical applications of placental extracts, cells, and tissue components is described. Empirically accumulated data are summarized and compared with modern research. Furthermore, we define scopes and outlooks of application of placental cells and tissues in the rapidly progressing field of regenerative medicine.